1. Field
The present specification generally relates to the manufacture of optical preforms, such as optical fiber preforms and, more specifically, to methods and apparatuses for forming optical preforms from glass soot.
2. Technical Background
Conventionally, the outer cladding portion of an optical fiber preform may be formed from an outside vapor deposition (OVD) process in which silica glass is deposited on a glass core blank, for example, through the pyrolysis of octamethyltetrasiloxane. The OVD process is a highly optimized, high yield manufacturing process. However, the formation of the cladding layer is often the rate limiting step in maximizing optical fiber output. Further, it is estimated that as little as 50% of the pyrolysis product of the octamethyltetrasiloxane feedstock is deposited on the glass core blanks during deposition of the outer cladding portion of the optical fiber preform. The remaining pyrolysis product of the octamethyltetrasiloxane feedstock is collected in a baghouse as silica glass soot. The collected silica glass soot has a relatively high purity, but still has contaminants which prevent it from being used to produce high quality claddings of optical fiber preforms. Attempts have been made to use a soot pressing process with this waste silica by employing a strenuous chemistry process to remove the impurities with mixed results. As a consequence, high purity soot generation processes have been developed on an industrial and lab scale permitting the soot characteristics (such as particle distribution and the index of refraction by the addition of dopants) to be altered at will.
Prior techniques for forming cladding layers of an optical preform from silica glass soot, such as vertical pressing, are only suitable for forming a single cladding layer. As such, these techniques cannot be utilized to form preforms that have a complex refractive index profile through the radial cross section of the preform. Specifically, these techniques cannot be used to reliably construct an optical preform which includes multiple layers of silica glass soot where each layer is formed from silica glass soot with a different composition and/or morphology.
Accordingly, a need exists for alternative methods and apparatuses for forming an optical preform, such as an optical fiber preform, from silica glass soot.